Multi-speed power transmissions, which employ mechanical clutches such as synchronizers, generally include pairs of meshing external gears to establish drive or speed ratios between a transmission input shaft and a transmission output shaft. These transmissions require a pair of meshing gears for each of the forward speed ratios, other than direct, and one and a half pair of gears for the reverse speed ratio.
Each of the gear pairs includes a synchronizer or mechanical clutch, which engages one of the gears with either the main shaft or a countershaft, while the other gear of that mesh is continuously driven by one of the other shafts. The synchronizers are generally two-way synchronizers such that each synchronizer clutch has a shift collar and an output member,. which can be connected with the shift collar individually. The synchronizers are then placed or situated between two of the gear pairs on a given shaft such that a single synchronizer having two output sides can control two gear meshes.
For example, a ten-speed transmission having a single reverse speed would require ten and a half pairs of gears, assuming one of the ratios is direct, and five and one-half synchronizers. The reverse ratio normally would include the input gear for first gear, an idler gear, and a separate output gear. Thus, the reverse ratio is relatively close in numerical value to the first ratio.